The present invention relates generally to the field of medical imaging and, more particularly, to the subject of diffusion weighted imaging using magnetic resonance techniques.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of magnetic resonance imaging (MRI), specialized radio frequency (RF) pulses are used to stimulate susceptible protons so that image information may be collected. Three axes, X, Y, and Z, are employed to acquire sufficient positional information about each proton to construct a three dimensional image. Further, each of the three axes is not uniform but comprises a magnetic gradient, allowing each proton to be measured relative to the axis by its position within the gradient. Absent the gradient, no meaningful positional information could be obtained. Use of specialized RF pulses and three gradient axes is common in MRI techniques.
Because MRI acquires information about susceptible protons, typically hydrogen protons, water and water containing fluids are a common imaging target. In the realm of medical imaging this allows the imaging of diffusion processes involving blood, cerebrospinal fluid, or other water containing bodily fluids. Because fluids are relatively mobile compared to other bodily tissues, special imaging techniques must often be employed.
The overall diffusion weighting, or b-value, describes the sensitivity of an MRI sequence to a diffusion process. In the most common diffusion encoding method, two large gradient pulses are separated by a 180 degree RF refocusing pulse. The b-value for this sequence is given by the Stesjkal-Tanner (S-T) equation:
b=(2xcfx80xcex3)2 g2 (xcex42(xcex94xe2x88x92xcex4/3)) 
where xcex3 is the gyromagnetic ratio, g is the amplitude of a diffusion lobe associated with the pulse sequence, xcex4 is the duration of a single diffusion lobe, and xcex94 is the interval between the start of a first diffusion lobe to the start of a second diffusion lobe. Typically an operator configures the MRI apparatus with a prescribed b-value to obtain a desired degree of sensitivity to diffusion processes.
However several factors influence the accuracy of the b-values calculated using this equation, and thereby the actual degree of sensitivity obtained in diffusion imaging. First, the S-T equation only considers square gradient waveforms. Square waveforms, however, cannot be achieved in practice due to the finite inductance of the gradient coil. In practice, gradient waveforms are instead trapezoidal, sinusoidal, or some other waveform shape which complicates determining the b-value analytically. These non-square waveforms creates deviations from the desired b-value when the S-T equation is used as an approximation. Therefore, configuring the degree of diffusion sensitivity is problematic, as is knowing the actual degree of diffusion sensitivity associated with an acquired diffusion image.
A second factor that influences the accuracy of the b-values calculated from the S-T equation are the imaging gradients used to localize the proton signal in space. The S-T equation does not take into account the contribution of the imaging gradients to the b-value. Since the b-value is used to calculate various diagnostic measures, such as Apparent Diffusion Coefficients (ADC) and diffusion anisotropy indices, the amount of error contributed by the imaging gradients is particularly important.
A third factor which contributes to the accuracy of the b-value and diffusion sensitivity in an image is the interactions between imaging gradients on each axis. These interactions lead to off-diagonal b-value terms and as a consequence, the diffusion weighting in a sequence is more accurately described by a b-matrix where the on-diagonal b-value terms represent the diffusion weighting along a particular axis and the off-diagonal b-value terms represent interactions between gradients across axes.
There is a need for techniques permitting more precise knowledge of b-values for use in MR imaging applications.
The present technique teaches the calculation of true b-values in an MR imaging system. This calculation is generally performed by determining linear segments between corner points of a gradient waveform. Integration of the linear segments is then performed and a true b-value is calculated from the sum.
Further steps may be performed to correct the MR imaging process, and particularly to correct the actual b-value to the desired b-value. Correction may be performed by means of a minimization routine or an iterative search with recalculation of the actual b-value performed after each iteration. Calculation and correction of the b-value is performed upon each axis of the gradient waveform.